Light-emitting diodes (LEDs) are attractive candidates for the replacement of conventional light sources based on incandescent and fluorescent lights. LEDs have significantly higher power efficiencies than incandescent lights and have much greater lifetimes. In addition, LEDs do not require the high voltage systems associated with fluorescent lights and can provide light sources that more nearly approximate “point sources”. The latter feature is particularly important for light sources that utilize collimating or other imaging optics.
LEDs emit light in a relatively narrow spectral band. Hence, to provide a light source of an arbitrary perceived color, the light from a number of LEDs must be combined in a single light fixture or some form of phosphor conversion layer must be used to convert the narrow band light to light having the desired color. While this complicates the construction of some LED light sources, it also provides the basis for light sources having a color that can be varied by altering the ratios of the light emitted by the various colored LEDs or an intensity by varying the power to all of the LEDs. In contrast, conventional light sources based on fluorescent tubes emit light of a fixed color and intensity.
A light source based on a single LED is relatively limited in the amount of light that the light source can generate. Typically, LEDs have power dissipations that are less than a few watts. Hence, to provide a high intensity light source to replace conventional light fixtures, a relatively large number of LEDs must be used in each light source.
In addition, LEDs age with use. Typically, the light output deceases with use and, in some cases, the spectrum emitted by the LED shifts with age giving rise to color shifts. In general, LEDs that emit different colors of light have different aging characteristics, since the aging profile of an LED depends on the fabrication process and materials, as well as other factors. In a light source based on three different color LEDs, the shift in intensity and/or spectrum causes the light emitted by the source to shift in color. To correct for these problems, many LED light sources include some form of photodetector that measures the light generated by the LEDs and adjusts the drive currents to each LED to maintain the desired color.
Most of the effort that has gone into designing LED light sources has been directed to overcoming the problems discussed above that prevent widespread use of the LED light source as replacements for conventional light sources. While the resultant designs have brought LED light sources closer to realizing their potential as replacements for conventional light sources, these devices have failed to take advantage of many of the other features inherent in LED light sources.